An important capability enhancement of Vertical Take-Off and Landing (VTOL) aircraft is the ability to transition from vertical flight to horizontal flight during take-off, and vice versa during landing. There have been many designs that attempt to achieve such capability. For example, the tail-sitter design, such as model no. XFV-1 of the 1950s, using the same set of flight controls for both vertical and horizontal flight, represents one of the most direct ways of achieving transition flight. However, with the pilot facing upwards during vertical flight, making visual assessments, e.g. during landing, can be difficult.
Also, tail-sitter aircraft have other technical issues. For example, they tend to be susceptible to toppling, e.g. when landing under windy conditions. This is due to a high centre of gravity, relative to the size of the tail base. Addressing this issue may involve installing landing gears of a wide span or enlarging the span of the tail base to cover a wider area on the ground. However, these measures usually add weight and aerodynamic drag, which in turn may compromise the performance (e.g. cruise endurance) of the aircraft.
One existing solution to the above problems comprises using configurations that remain horizontal during transition, e.g. tilt-wings and/or tilt-rotors. This may also render the aircraft suitable for carrying passengers. However, tilt-wings and tilt-rotors need separate sets of flight controls for helicopter-mode and airplane-mode flight, resulting in high complexity in their development and implementation.
Unmanned aircraft, also known as unmanned aerial vehicles (UAVs), on the other hand, do not carry passengers or pilots. Hence, passenger- and pilot-related limitations of tail-sitters are not applicable to the design of transition-capable VTOL UAVs.
However, there are other issues which may arise during the development of autonomous flight transition for an UAV. For example, a typical transition manoeuvre spans a wide range of airspeeds and angles-of-attack. The presence of variables which cover a wide range of values, when multiplied in combinations with the other variables, can potentially result in massive aerodynamic databases for adequate coverage of the transition envelope. This may require significant effort and cost to generate by means such as wind tunnel testing, computational fluid dynamics (CFD) etc., when developing autonomous transition. In addition, highly non-linear aerodynamic characteristics and changes in stability characteristics associated with higher angles-of-attack require complex, non-linear control strategies and algorithms to be developed, further adding to the complexity of development efforts.
WO2013/048339 discloses a quad-rotor UAV capable of vertical and horizontal flight modes. However, in the event that one of the propellers fails, a loss of controllability of the UAV may occur.
A need therefore exists to provide a UAV that seeks to address at least some of the above problems.